Jaeger Exporter Bugs

[CodeBlanch]

Working on open-telemetry/opentelemetry-dotnet-contrib#38 I ran into this issue where none of my http client spans were making it to Jaeger. I tracked this down to a null reference exception being thrown in Jaeger calling ToString() on null tag values (you can still get nulls using Activity.AddTag syntax). While I was fixing that I noticed that we support certain arrays for tag values on Activity but not on ActivityEvent. I fixed that up too.

[reyang]

LGTM.

[codecov]

Codecov Report

Merging #1310 into master will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1310      +/-   ##
==========================================
- Coverage   78.56%   78.51%   -0.05%     
==========================================
  Files         219      219              
  Lines        6237     6243       +6     
==========================================
+ Hits         4900     4902       +2     
- Misses       1337     1341       +4     

Impacted Files	Coverage Δ
....Jaeger/Implementation/JaegerActivityExtensions.cs	95.73% <100.00%> (+0.16%)	⬆️
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	77.77% <0.00%> (-3.18%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	61.76% <0.00%> (-2.95%)	⬇️